The present invention relates to a dynamic transmission power control method, and more particularly, to a dynamic transmission power control method for a transmitter of a powerline communication system capable of generating an optimal power cutback level for each receiver, to effectively maximize throughput and increase power efficiency without inducing hidden-node issue.
Transmission power back-off technology has been proposed to either increase throughput or reduce power consumption or electromagnetic radiation. However, determining the transmission power back-off level has been a challenge in communication systems that could suffer from the hidden-node problem.
Specifically, since the signal to noise ratio (SNR) dynamic range of an Analog-to-Digital (A/D) converter at a receiver is limited, the transmission power spectral density (PSD) adjustment, the aggregate transmission power adjustment or the gain scaling adjustment will generate better SNR at receiving side at some parts of subcarriers. Accordingly, receiver's throughput will be increased.
For example, please refer to FIG. 1A to FIG. 1B. FIG. 1A is a schematic diagram of a non-flat transmit PSD mask of a powerline communication (PLC) system, wherein PSD masks corresponding to different frequency bands are −55, −85 and −120 dBm/Hz, respectively. FIG. 1B is a schematic diagram of SNR of signals received by a receiver of the PLC system when utilizing the non-flat transmit PSD mask shown in FIG. 1A. As shown in FIG. 1A, in order to comply with the regulation of a country, the transmit PSD mask of the PLC system may not be flat, and the PSD mask for some active subcarriers could be lower than that for the other active subcarriers, e.g. the PSD mask of a high frequency band is lower than the PSD mask of a low frequency band. In certain cases without transmit PSD adjustment as shown in FIG. 1A, an analog automatic gain control (AGC) setting on the receiver can not drive the channel noise above the quantization noise level of an A/D converter for all frequency tones due to limited dynamic range of the converter, and thus those subcarriers with lower reference PSD have lower SNR as shown in FIG. 1B (lower than 25 dB). In other words, since signals in the low frequency band have high transmission power and thus the AGC can only provide a low gain to prevent saturation of the A/D converter, signals in the high frequency band with low transmission power can not be amplified by the AGC with a high gain and thus have low SNR.
On the other hand, please refer to FIG. 1C and FIG. 1D. FIG. 1C is a schematic diagram of a non-flat transmit PSD mask of the PLC system applied with low-band transmission power back-off, wherein PSD masks corresponding to different frequency bands are −65, −85 and −120 dBm/Hz, respectively. FIG. 1D is a schematic diagram of SNR of signals received by the receiver of the PLC system when utilizing the non-flat transmit PSD mask shown in FIG. 1C. As shown in FIG. 1C, if low-band transmission power back-off is applied (10 dBm lower), SNRs of those subcarriers with lower reference PSD masks can be improved significantly (10 dB higher) In other words, if transmission power of signals in the low frequency band is reduced, the AGC can provide higher gain without saturation of the A/D converter, and thus signals in the high frequency band can have higher SNR.
As can be seen from the above, the transmitter needs to know a power back-off level that maximizes the benefit of transmission power back-off. However, careless transmission power back-off may result in the hidden-node problem that an on-going packet may be interfered by some distant nodes which cannot hear the signal from the packet transmitter since transmission power of the signal from the packet transmitter is reduced too much for the distant node to hear due to path loss.
For example, please refer to FIG. 1E, which is a schematic diagram of a basic service set (BBS) 10 of a PLC network. As shown in FIG. 2, the BBS 10 includes a BSS manager BM1 and stations A-D, wherein the stations A-D are all associated with the BSS manager BM1 (also a station). Under such a configuration, when the station A transmits packets to the station B, a “hidden node” issue may occur, i.e. some stations (e.g, the station C, D, or the BSS manager BM1) in the BSS 10 may be not aware that the station A is transmitting packets to the station B since the station A performs a careless transmission power back-off and the station C, D, or the BSS manager BM1 can not detect the packets transmitted from the station A to the station B.
Therefore, since careless transmission power back-off may result in the hidden-node problem that an on-going packet may be interfered by some distant nodes which can not hear the signal from the packet transmitter, there is a need to improve over those prior arts.